Thin films that protect an underlying material sensitive to moisture, oxygen and other chemicals are commonly used in many industrial processes. Applications range from food packaging, hard coatings on eyeglass lenses, and window glass, to protection of integrated circuits, display screens, and photovoltaic panels. Such thin films need to be dense, have excellent adhesion to the underlying layer and not crack or peel for the life of the product. In some cases the coating needs to be a hermetic seal, keeping out water vapor and oxygen, and in this case it must not have too many tiny pinhole leaks that would disrupt the function of even tiny areas of the underlying structure or device. Further, for many of the most high value applications such as OLED display screens, lighting panels, or organic photovoltaic panels, where the barrier needs to be tightest, the under-layers must not be exposed during manufacturing to temperatures above a limit that may range from slightly over hundred degrees Celsius in some cases to less than about seventy five degrees for some polymers. In some new products such as flexible circuits and screens, the device that needs to be protected will be built on a flexible substrate material and must be capable of being bent repeatedly without causing the protective function to be compromised.
Presently, adhesion of deposited coatings is achieved either by first putting down an intermediate layer of highly wettable polymer layers, or by subjecting the surface to an inert plasma. Where the application is very cost sensitive it may be too expensive to use such wettable polymers, and for many inexpensive plastics an inert gas treatment has been found not to be effective in promoting adequate adhesion of the hard coating. Therefore, a more effective and less expensive method of ensuring adequate adhesion of coatings is needed.
For deposition of dense hermetic barrier layers at such low temperatures, sputtering of target material onto the substrate is the most common method used. This technique works quite well at substrate temperatures less than or about 100° C., but this technique generates substantial heat and produces films that are often not as amorphous and effective as barriers. In some applications where the plastic or polymer substrate is thick or cannot be cooled effectively sputtering may not be acceptable due to heating of the substrate. Plasma enhanced CVD has been used very predominantly in applications where the limiting temperature for the substrate is above about 250° C., but has not been capable of providing commercially competitive rates of deposition of high quality dielectrics at substrate temperatures under 100° C.
There are a few new and very demanding, high value applications for the hermetic encapsulation processes. Among these are encapsulation of organic and CIGS photovoltaic devices (PV), and encapsulation of organic light emitting diode (OLED) devices for both lighting and displays (Active Matrix OLED=AMOLED). These applications all have the strong requirement that the encapsulation be highly transparent to visible light, and very low in moisture and oxygen penetration. Solar panels using thin film materials such as CIGS or organic polymer for photovoltaic conversion require encapsulation with transmission rates from 10−4 to 10−5 gm/m2-day of water vapor. For these applications cost must be very low as well so that end products are competitive. For applications such as OLED lighting and Organic PV modules the cost per square meter should be less than or about US$10/m2 since the total cost of such panels or web needs to be less than US$60/m2 and even as low as US$30/m2. For CIGS encapsulation cost must be no greater than $15/m2 to $20/m2, and for OLED displays encapsulation cost may be as much as $100/m2 since the display screen total cost of manufacture will be between about $1000/m2 to $2000/m2.
The most demanding application is for AMOLED displays. Compared to commonly used liquid crystal displays (LCD), AMOLED technology can provide many benefits, including lower power consumption, higher contrast, wider viewing angles and the ability to be made on flexible substrates. But there are also substantial technological challenges to be solved before AMOLED displays larger than a square decimeter—such as useful for tablet or laptop displays—can be manufactured with high yield.
In particular, the interface between low work-function metals, used for the electron emitting layer in an OLED device, and the electron transport layer are highly sensitive to damage by oxidation. Therefore, to achieve a useful lifetime in air, an OLED display must be encapsulated such that the oxygen transmission rate (OTR) is less than 10−3 to 10−5 scc/m2-day and water vapor transmission rate (WVTR) is even less than 10−7 g/m2-day with NO pinholes whatever! Currently this can only be done in mass production using a top covering of glass which is 100 or more microns thick. In comparison an LCD display is relatively insensitive to water or oxygen and requires encapsulation rated at OTR and WVTR of order 0.1 scc/m2-day or g/m2-day.
As a reference point to understand the needed tightness of such encapsulation, the air and moisture leakage requirement for an OLED display is equivalent to that of a high-vacuum chamber with a He leak rate on the order of 10−10 standard cc/sec. High-vacuum chambers with such a high degree of vacuum integrity are not uncommon, but require careful design, use metal seals, are expensive to make and are not generally mass-produced.
It has been demonstrated that OLED displays can be sufficiently encapsulated when built on glass substrates by installing a top glass layer with a perimeter seal to the OLED area. As this perimeter seal is based on polymers that allow for some permeation or leakage of oxygen and water, requiring a “gettering” material in the space surrounding the OLED to absorb oxygen and water. This is an expensive technique ($50/m2 to $100/m2) and only suitable for relatively small and rigid displays, such as on smart phones or tablets. This encapsulation technique also suffers from difficulty relating to stresses when front and back surfaces are not maintained at precisely the same temperature. Finally, the double glass encapsulation method is almost inflexible, even when the glass cover is extremely thin. This is extremely limiting since the highest value OLED displays will be those that are flexible, both because they will be lighter and less fragile, and able to made into new and compact displays that simply cannot be made with LCD technology. Thus, new encapsulation methods are needed for OLED to realize its full potential.
More recently, other types of “thin film” hermetic coatings made with plasma-based CVD methods have been tried. In particular they have been inorganic dielectrics such as silicon dioxide, silicon nitride, and other materials often made by Plasma enhanced or assisted deposition methods or atomic layer deposition (ALD) of such materials as aluminum oxide and titanium dioxide. Such films when they are thick enough to be robust and scratch resistant are quite brittle and do not tolerate flexure of the substrate very well. Further, thicker conventional PECVD or PACVD layers that are more robust have poor bonding around the edges of the surface defects and during flexure of the substrate initiate microcracks around such defects where material quality is poor. Such microcracks then propagate into the barrier layer resulting in leakage paths for moisture.
To reduce cost, increase manufacturing yield, and increase applications for OLED both in lighting and displays there is a need to find methods and tools that enable high-volume production encapsulation with transparent thin films between about 30 nm and about 10 μm thick that provide the equivalent integrity of a high-vacuum chamber. In the case of flexible displays, which would be useful for many commercial applications, hard, inorganic barriers usually need to be somewhat less than about 100 nm thick to avoid cracking when the screen is flexed or there is a temperature difference between front and back surfaces of tens of degrees Celsius. The consequent leaking of atmosphere into the sensitive material layers damages the device, making a “black spot” on the screen or lighting panel. Other barrier materials may be mixed organic-inorganic coatings that have both high transparency for visible light and barrier function while being up to 10 microns thick.
The prior art demonstrates that thin barrier films exist that have the ability to meet the requirements of OLED encapsulation under ideal conditions. Films including of inorganic nitrides, oxides, and oxynitrides are particularly suitable as they also are transparent. In particular aluminum oxide, silicon nitride and silicon oxynitride are commonly used. These are highly transparent and yet very dense dielectrics that do not permit substantial moisture permeation and can be applied by vacuum coating processes for which there is substantial production experience.
Some prior art overcomes the effect of localized particles or other defects in one barrier layer by using multiple such layers, often with a planarizing, soft and flexible organic inter-layers between inorganic barrier layers. In the case of the organic inter-layers the motivation is to bury the defects in the flexible polymer and deposit each new barrier layer on the clean polymer surface. This ideally results in a wide lateral separation of the defects in successive barrier layers such that the effective path length for transmission of oxygen and water molecules is substantially increased, and a very long latency period before moisture can penetrate all layers to the device. The prior art suggest that as many as 3 to 7 repeated stacks of interlayer and barrier films are required to achieve oxygen or water vapor transmission rates (OTR & WVTR respectively) adequate for extended lifetime (up to 10 years) of an OLED display.
Panels, modules, or sheets of organic PV or CIGS are more cost sensitive than AMOLED display applications and therefore cost effective thin film encapsulation can be an even more important enabler of the cost reductions that are essential for their competitiveness in the energy conversion marketplace. Currently, the cost of making PV panels is roughly US$1 per Watt so that panel cost is roughly $100 to $150 per square meter. The encapsulation cost should therefore be no more than about 10% to 15% of this and yet must last for at least 10 years and probably more than 20 years. Since the panels produce the most electricity when exposed directly to sunlight it is likely that most of these panels must be able to withstand exposure to the elements and dust, and large ranges of temperature (−10° C. to 80° C.). It is also essential that the encapsulation be able to expand to accommodate the substantial thermal expansion of the substrate. While very thin (<30 nm) inorganic films such as silicon dioxide and silicon nitride accommodate substantial expansion without cracking, thicker films often do crack.
Since the efficiency of such panels is critical to their cost-effectiveness, the panels would also strongly benefit by having anti-reflection coatings that could make the panels more efficient by reducing reflected light. Further, it would be helpful if an antireflection coating had an ability to resist scratching so that cleaning dust would not reduce the light transmission and efficiency. Such cleaning must be done several times a year to avoid efficiency loss. The sum of costs for all the above different coatings beneficial to the PV function should stay within the cost limits roughly of US$15 to US$20 per square meter. There are currently no known deposition processes that can produce the coatings within the cost limits. If such a process was found it would give an enormous boost to OPV and CIGS and to PV technology in general.
Therefore, for AMOLED displays, OLED lighting, and possibly some PV technologies a need for deposition methods for thin barrier films that have suitable bulk properties for low oxygen and water transmission rates. It is further necessary that such methods ensure excellent adhesion of encapsulation to the underlayers, and avoid formation of local defects due to imperfections in the starting surface. It is further necessary that the number of defects per square meter be of order 1 or less, and that the coating tolerate flexure—in some cases with very small bending radius—without micro-cracking, losing barrier function or peeling.